Rapid technological advances have been made in various fields such as energy conservation, information processing and life science. In accordance with these advances, materials, especially ceramics which constitute the basis of the technology, have been required to have higher quality and performance. Need for development of materials with new and excellent properties and functions have had high priority recently.
Among the various needs, it is especially important to manufacture dense, homogeneous and fine-grain ceramics with high reliability at relatively low temperature.
Many ceramics such as Al.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2 exist in a variety of crystalline forms or polymorphs. Transformation of the polymorphic forms of a ceramic are classified as either displacive or reconstructive, depending on the extent of atomic reorganization necessary for the transformation. In general ceramic materials that undergo reconstructive transformation (e.g. Al.sub.2 O.sub.3, TiO.sub.2), transform by nucleation and growth and have high activation energies. Most of the activation energy of a transformation is utilized in the nucleation process. Thus, elevated temperatures are required to overcome the high activation energy barrier for nucleation.
For ceramic fabrication it is necessary to transform the ceramic powder to the stable form before consolidation, because low densities are usually obtained on sintering if the ceramic powder undergoes transformation during heating. However, because of the elevated temperatures imposed by the high activation energy, the ceramic powder becomes coarsened, aggregated and generally unsuitable for ceramic processing and fabrication without additional intermediate processing, prior to the ceramic processing and fabrication.
Many technological as well as scientific advances have been achieved in the ceramic field based on need. As a representative example, a process involving low temperature preparation of non-agglomerated monosized powders with high purity, consolidation of the powders using a colloidal process, and sintering them at a lower temperature than usual has been extensively studied.
Such a process would be very attractive not only because of the possibility of circumventing problems associated with conventional processes and their products, such as non-homogeneity, grain coarsening and low reliability, but also because of energy conservation. One of the problems of such a process, however, is that low temperature preparation of ceramic powders usually results in the formation of powders in metastable phases different from the phases of final sintered bodies. For example, amorphous or crystalline aluminum hydroxides are commonly obtained by low temperature synthesis of aluminum oxide and alpha alumina (alpha alumina is the thermodynamically stable phase of aluminum oxide in the usual atmosphere but whose synthesis at low temperature has not been reported).
Low temperature synthesis of TiO.sub.2 and 2RO.sub.2 usually leads to the formation of amorphous or metastable crystalline phases.
Although it is feasible to convert powders in metastable phases into stable phases by appropriate heat treatment, problems are encountered such as high energy consumption, complexity of the process and property degradation of the powders (extraordinary grain growth, aggregation, etc.) during the treatment.
On the other hand, when powders in the metastable phases are used as starting materials for sintered ceramics without treatment, deleterious microstructural changes are commonly observed in the sintered bodies during transformation, typically from a metastable to a stable phase.
The microstructural change is usually accompanied by rapid grain growth and formation of closed pores inside the grains. As a result, this transformation results in non-homogeneous sintered bodies with entrapped pores and coarse grains. A higher temperature is also required to obtain a dense, sintered body. These disadvantages are further aggravated by presence of adsorbed water and gases, which tend to leave many closed pores in the sintered bodies during the densification process when fine powders prepared at low temperature are used as starting materials.
Mainly because of these problems many processes using fine powders prepared at low temperatures have not achieved what is believed to be their potential, namely both excellent performance and high reliability in a wide range of ceramic materials.